Competition between photochemistry and energy transfer in ultraviolet-excited diazabenzenes. I. Photofragmentation studies of pyrazine at 248 nm and 266 nm

The quantum yield for the formation of HCN from the photodissociation of pyrazine excited at 248 nm and 266 nm is determined by IR diode probing of the HCN photoproduct. HCN photoproducts from excited pyrazine are produced via three different dissociation channels, one that is extremely prompt and two others that are late. The total quantum yield from all reaction channels obtained at low quencher gas pressures, phi = 1.3 +/- 0.2 for 248 nm and 0.5 +/- 0.3 for 266 nm, is in agreement with preliminary studies of this process as well as recent molecular beam studies. To investigate if HCN production is the result of pyrazine multiphoton absorption, this photodissociation process has been further studied by observing the HCN quantum yield as a function of total quencher gas pressure (10 mTorr pyrazine, balance SF(6)) and as a function of 248 nm laser fluence from 2.8 to 82 mJ/cm(2). At the highest SF(6) pressures, the HCN quantum yield shows strong positive correlation with laser fluence, indicating that the prompt channel is the result of multiphoton absorption; however, at low pressure, the HCN quantum yield is affected little by changing laser fluence, indicating that the majority of the HCN photoproducts at low pressure are produced from pyrazine which has absorbed only one UV photon. At the lowest pressures sampled, HCN produced from the one-photon late process accounts for more than 95% of all HCN formed (at low laser fluence). At high pressures the single photon late pyrazine dissociation is quenched, and HCN produced at high quencher gas pressures comes only from the multiphoton absorption channel, which can be clearly observed to depend on laser fluence. The HCN quantum yield as a function of laser intensity at high pressure has been fit to a quadratic function that can be used to determine the amount of prompt unquenched HCN produced from multiphoton photodissociation. Additionally, the information theoretic prior functions for energy disposal in the 248 nm photodissociation of pyrazine to form HCN have also been developed. Prior functions for one, two, and three-photon absorption indicate that only HCN with near room temperature translational energy comes from the one-photon process and that all HCN molecules with large amounts of translational energy are produced by multiphoton processes. Finally, analysis of the quenching data within the context of a strong collision model allows an estimate of the rate constant for HCN production from pyrazine for the major late channel, k(d1s) = 1.69 x 10(5) s(-1), for 248 nm excitation, and k(d1s) = 1.33 x 10(4) s(-1) for 266 nm excitation. After 266 nm excitation, pyrazine produced by the major one-photon channel lives for almost an order of magnitude longer than after 248 nm excitation. (C) 2000 American Institute of Physics. [S0021-9606(00)00310-X].